Jackson中处理map中的null key 或者null value 及实体字段中的null value

1.map中有null key时的序列化

 当有null key时，jackson序列化会报 Null key for a Map not allowed in JSON (use a converting NullKeySerializer?) 

处理此异常有两种方式

• 1.需要自定义一个序列化null key的方法
• 2. map中直接remove null key

这里只讨论第一种：

处理方法为 mapper.getSerializerProvider().setNullKeySerializer(new NullKeySerializer()); 将null key处理为空字符

    @Test
    public void testSerializMapNullKey() throws JsonProcessingException {
        //ignore map null value
        Map<String, Object> map = new HashMap<>();
        map.put(null, "test");
        ObjectMapper mapper = new ObjectMapper();
        mapper.getSerializerProvider().setNullKeySerializer(new NullKeySerializer());
        System.out.println(mapper.writeValueAsString(map));

    }

static class NullKeySerializer extends StdSerializer<Object> {
        public NullKeySerializer() {
            this(null);
        }

        public NullKeySerializer(Class<Object> t) {
            super(t);
        }

        @Override
        public void serialize(Object nullKey, JsonGenerator jsonGenerator, SerializerProvider unused)
                throws IOException, JsonProcessingException {
            jsonGenerator.writeFieldName("");
        }
    }

{"":"test"}

Process finished with exit code 0

2. 处理null value的情况

    @Test
    public void testIgnoreMapNullValue() throws JsonProcessingException {
        //ignore null key
        Map<String, Object> map = new HashMap<>();
        map.put("key", "test");
        map.put("key1", null);
        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        System.out.println(mapper.writeValueAsString(map));

    }

输出为：{"key":"test"} 并没有 key1:null

{"key":"test"}

Process finished with exit code 0

3. 处理实体中filed中值为null的情况

使用注解：@JsonInclude(JsonInclude.Include.NON_NULL)

注意：@JsonInclude(JsonInclude.Include.NON_NULL)即可以在实体上用，也可以在filed中使用，比如在name上用这个注解

    @Test
    public void testIgnoreNullFiled() throws JsonProcessingException {
        //test ignore null filed
        User user = new User();
        user.setName(null);
        user.setHouse("asdf");
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.writeValueAsString(user));

    }

    /**
     * ignore null filed
     */
    @JsonInclude(JsonInclude.Include.NON_NULL)
    class User {
        private String name;
        private String house;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getHouse() {
            return house;
        }

        public void setHouse(String house) {
            this.house = house;
        }
    }

输出为：

{"house":"asdf"}

Process finished with exit code 0

如果设置全局怎么设置呢，使用： mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL)；

    @Test
    public void testIgnoreNullFiledGlobally() throws JsonProcessingException {
        //test ignore null filed
        User user = new User();
        user.setName(null);
        user.setHouse("asdf");
        ObjectMapper mapper = new ObjectMapper();

        //Ignore Null Fields Globally
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        System.out.println(mapper.writeValueAsString(user));

    }

参考：1.https://www.baeldung.com/jackson-map-null-values-or-null-key

         2.https://github.com/eugenp/tutorials/tree/master/jackson-simple